In actuators of the type having a diaphragm or piston whose linear movement is translated into rotary movement of an output member and vice versa for pressure produced or pressure producing action respectively by the diaphragm or piston, it is known to employ a helical coupling therebetween for effecting such motion conversion. Various designs of a helical coupling for effecting such motion conversion have been proposed but they are normally complicated in design and require many parts and considerable time in their assembly.